Fix a bug in determining if yarn is available

Reviewed By: mkonicek

Differential Revision:
D4276707
Ninja: OSS

fbshipit-source-id: 79cb2942f27179681d1a8ebfd3bc5a97f544f7af

local-cli/util/yarn.js

@@ -8,6 +8,7 @@
  */
 'use strict';
 
+const execSync = require('child_process').execSync;
 const fs = require('fs');
 const path = require('path');
 const semver = require('semver');